Add a deprecation message to rtcpMuxPolicy of negotiate.

third_party/WebKit/Source/core/frame/Deprecation.cpp

Index: third_party/WebKit/Source/core/frame/Deprecation.cpp
diff --git a/third_party/WebKit/Source/core/frame/Deprecation.cpp b/third_party/WebKit/Source/core/frame/Deprecation.cpp
index f5bf16d3ed48712a71c82f117d4ba626b1cf48ad..4b8302afe2207291798c6e012b05b78f9eb9d032 100644
--- a/third_party/WebKit/Source/core/frame/Deprecation.cpp
+++ b/third_party/WebKit/Source/core/frame/Deprecation.cpp
@@ -430,6 +430,14 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature) {
           "as of %s. See https://www.chromestatus.com/features/5030265697075200"
           " for more details.", milestoneString(M59));
 
+    case UseCounter::RtcpMuxPolicyNegotiate:
+      return String::format(
+          "The rtcpMuxPolicy option is being considered for "
+          "removal and may be removed no earlier than %s. If you depend on it, "
+          "please see https://www.chromestatus.com/features/5654810086866944 "
+          "for more details.",
+          milestoneString(M60));
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
       return String();